Neglected cellular organelles.
Virtually all animal cells contain organelles called glycosomes
which until now have been neglected in cell biology. This author
recently published a comprehensive review article entitled "Glycosomes -
the organelles of glycogen metabolism", Tissue & Cell 28 (3) 253-265,
1996. The review, based on biochemical and microscopy data, includes a
complete history of glycogen research and the current status of knowledge.
Biochemical studies on glycogen metabolism demonstrate the
existence of a complex enzymatic machinery involved in glycogen synthesis
and degradation, which constantly accumulates glycogen and releases
glucose. Such types of cellular machineries represent cell organelles and
these were already called glycosomes. There is, however, a confusion in
the morphological data concerning glycosomes since early electron
microscopic research incorrectly identified the protein component of
glycosomes as particles of glycogen. This interpretation, accepted in
textbooks and diagnostic pathology, needs an urgent revision. The
protein component in glycosomes is fixed by osmium, and stained by uranium
and lead. Glycogen accumulated in glycosomes is neither fixed per se nor
stained by heavy metals. Glycogen can be visualized by special
techniques such as histochemistry or negative staining. The differences
in size and in the electron density of glycosomal protein stained by
uranium and lead (commonly called Ôglycogen particlesÕ) indicates the
metabolic state of glycosomes rather then the amount of glycogen. Several
data suggest that the large, electron dense particles may appear when
phosphorylase is present (or active) in the organelle, whereas the small
and poorly contrasted protein particles would imply the activity of
glycogen synthase.
Furthermore, there is an intimate association between glycosomes
and numerous other cellular organelles including the intermediate
filaments and the membranes of endoplasmic reticulum. The association may
be related to the energy release by glycosomes, as well as to the
transport of glycosomes within the cell, similar to the well recognized
transport of ribosomes. The understanding of the structure of glycosomes
opens a vast field for the application of modern molecular and cellular
biology techniques in order to study cellular metabolism of glycosomes and
the role of these ubiquitous organelles in the cell.
I would be very happy to receive some feedback on the ideas I
outlined in my review article and to continue discussion with anyone
interested in glycogen research and in the integrated research on the
cell.
Please, reply via e-mail: rybicka at acsu.buffalo.edu
Krystyna Kielan Rybicka
